Two-component polyurethane clear coat kit system

ABSTRACT

A two-component polyurethane clear coat kit system consisting of:
     (1) a binder component having a solids content of 42 to 50 wt.-% comprising at least one hydroxyl-functional binder and a volatile organic content of 50 to 58 wt.-%, and   (2) a polyisocyanate crosslinker component having a solids content of 66 to 70 wt.-% and a volatile organic content of 30 to 34 wt.-%, wherein the solids content of the polyisocyanate crosslinker component comprises a free polyisocyanate solids content consisting of 75 to 100 wt.-% of at least one polyisocyanate of the 1,6-hexane diisocyanate isocyanurate type and of 0 to 25 wt.-% of at least one polyisocyanate of the isophorone diisocyanate isocyanurate type, wherein the sum of the respective wt.-% in each case totals 100 wt.-%.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application Ser. No. 61/167,308, filed Apr. 7, 2009, whichis hereby incorporated by reference in its entirety.

FIELD OF THE PRESENT INVENTION

The present invention relates to a two-component polyurethane clear coatkit system the components of which can be statically mixed to form atwo-component polyurethane clear coat composition which can be used in aprocess for the preparation of a clear coat layer of an automotive OEM(original equipment manufacture) multi-layer coating.

BACKGROUND OF THE PRESENT INVENTION

Two-component polyurethane coating compositions are coating compositionsthat crosslink by formation of urethane bonds as a result of theaddition reaction between the hydroxyl groups of a hydroxyl-functionalbinder component and the free isocyanate groups of a polyisocyanatecrosslinker component. The hydroxyl-functional binder component and thepolyisocyanate crosslinker component are stored separately from oneanother before being mixed to form the two-component polyurethanecoating composition. Typically, two-component polyurethane coatingcompositions are prepared just prior to their application by mixing ahydroxyl-functional binder component with a polyisocyanate crosslinkercomponent. Typical mixing equipment for two-component coatings presentin automotive OEM coating plants are static mixers such as, inparticular, Kenics mixers.

In automotive OEM coating plants the substrates to be spray coated aresupplied in succession to the spray application apparatus, for example,by using an automatic conveying apparatus, for example, a conveyor belt.The spray application itself is effected while the individual substrateto be spray coated and/or the spray application apparatus are in motion.

The spray application process in automotive OEM coating plants isdistinguished by breaks or interruptions which do not only occur betweentwo individual substrates, i.e. not only in the time period after thespray coating of a substrate has been finished and before that of thefollowing substrate is started, but even in the course of the spraycoating of an individual substrate. Such breaks or interruptions mayhappen either unintentionally or deliberately and they may differ induration. For example, they may take 0.5 seconds to 15 minutes. Duringsuch breaks or interruptions no coating material is sprayed and, afterthe break has ended, spray coating is taken up again at that position onthe substrate surface where it had been interrupted when the breakbegan.

Whereas such positions on a substrate surface as described in thepreceding paragraph cannot be visually perceived while a clear coatlayer applied from a two-component polyurethane clear coat is still wetor uncured, it may happen that such positions turn up as unwantedvariations in optical appearance, after the clear coat has been bakecured. Examples of such variations in optical appearance are inparticular visually perceptible mattings. Such optical surface defectsmean a need for reworking or repainting and lead to productivity losses.In this respect there is a desire for a more robust automotive OEMtwo-component polyurethane clear coat system which allows to minimize oreven eliminate the occurrence of said unwanted optical surface defectsand the reworking or repainting effort associated therewith.

It has been found that said desire can be satisfied by providing atwo-component polyurethane clear coat kit system consisting of twocomponents having a certain composition.

SUMMARY OF THE PRESENT INVENTION

The present invention relates to a two-component polyurethane clear coatkit system consisting of:

-   (1) a binder component having a solids content of 42 to 50 wt.-%    (weight-%) comprising at least one hydroxyl-functional binder    (binder with hydroxyl groups) and a volatile organic content of 50    to 58 wt.-% (hereinafter for brevity purposes also called “binder    component”) and-   (2) a polyisocyanate crosslinker component having a solids content    of 66 to 70 wt.-% and a volatile organic content of 30 to 34 wt.-%,    wherein the solids content of the polyisocyanate crosslinker    component comprises a free polyisocyanate solids content consisting    of 75 to 100 wt.-% of at least one polyisocyanate of the 1,6-hexane    diisocyanate isocyanurate type and of 0 to 25 wt.-% of at least one    polyisocyanate of the isophorone diisocyanate isocyanurate type,    wherein the sum of the respective wt.-% in each case totals 100    wt.-%.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The binder component and the polyisocyanate crosslinker component of thetwo-component polyurethane clear coat kit system of the presentinvention can be statically mixed to form a two-component polyurethaneclear coat composition which can be used in a process for thepreparation of a clear coat layer of an automotive OEM (originalequipment manufacture) multi-layer coating.

The binder component has a solids content of 42 to 50 wt.-% comprisingat least one hydroxyl-functional binder and a volatile organic contentof 50 to 58 wt.-%, wherein the sum of the wt.-% totals 100 wt.-%.

In the description and the claims the terms “binder solids” and“crosslinker solids” are used. They refer to the solids contribution ofthe coating binders (binder solids) and the solids contribution of thecrosslinkers (crosslinker solids) contained in the two-componentpolyurethane clear coat composition which can be obtained by staticallymixing the binder component and the polyisocyanate crosslinker componentof the two-component polyurethane clear coat kit system of the presentinvention (hereinafter also referred to as “two-component polyurethaneclear coat composition”). Binder solids and crosslinker solids togetherform the resin solids of the two-component polyurethane clear coatcomposition. The resin solids together with any further non-volatileconstituents that may be present in the two-component polyurethane clearcoat composition form the solids of the two-component polyurethane clearcoat composition. Examples of such further non-volatile constituentsinclude non-volatile additives and transparent fillers (transparentextender pigments). To avoid misunderstandings, the solids content ofthe two-component polyurethane clear coat composition shall not beconfused with the solids content of the binder component or of thepolyisocyanate crosslinker component.

In the description and the claims the term “volatile organic content” isused. It refers to the sum of all volatile organic constituents, i.e,the sum of organic solvents and, if present, volatile organic additives.

Apart from the volatile organic constituents and the at least onehydroxyl-functional binder, the binder component may also comprise oneor more of the following optional constituents: one or more furtherbinders other than (other than=different from) hydroxyl-functionalbinders, one or more further crosslinkers other than free polyisocyanatecrosslinkers, non-volatile additives and transparent fillers. The solidscontent of the binder component has a hydroxyl number originating fromthe at least one hydroxyl-functional binder of, for example, 110 to 160mg KOH/g.

Both, the binder component as well as the two-component polyurethaneclear coat composition comprise one and the same binder solids. Thebinder solids comprise the at least one hydroxyl-functional binder andthe optionally present further binders other than hydroxyl-functionalbinders. In a particular embodiment, the binder solids consist of one ormore hydroxyl-functional binders.

Hydroxyl-functional binders and methods for their preparation are knownto the person skilled in the art of paints and coatings. Examplesinclude hydroxyl-functional resins like polyurethane resins,(meth)acrylic copolymer resins and polyester resins, but also polymerhybrid resins, wherein two or more of said resin types bound by covalentbonds or in the form of interpenetrating resin molecules are present.(Meth)acryl or (meth)acrylic is to be understood, both here and in thefollowing, as acryl and/or methacryl or as acrylic and/or methacrylic.Hydroxyl-functional (meth)acrylic copolymer resins and polyester resinsare preferred as hydroxyl-functional binders in the binder component ofthe two-component polyurethane clear coat kit system of the presentinvention. The hydroxyl-functional binders are oligomeric or polymericcompounds with a number-average molar mass (Mn) in the range of, forexample, 500 to 5000, preferably 1000 to 3000. Their hydroxyl numbersare in the range of, for example, 100 to 300 mg KOH/g.

All statements made in the present description and the claims inrelation to number-average molar masses relate to number-average molarmasses determined by GPC (gel permeation chromatography, polystyrenestandards, polystyrene gel as stationary phase, tetrahydrofuran asmobile phase).

Examples of further binders other than hydroxyl-functional binders whichmay be contained in the binder component include binder resins withoutfunctional groups or with functional groups other than hydroxyl groups.Examples of resin types are the same as those mentioned above for thehydroxyl-functional binders. The further binders belong to the bindersolids content of the binder component and they may form up to 10 wt.-%of the binder solids content. It is however preferred that the bindercomponent contains no further binders other than hydroxyl-functionalbinders.

Examples of crosslinkers other than free polyisocyanate crosslinkerswhich may be contained in the binder component include reversiblyblocked polyisocyanate crosslinkers and aminoplast resins, inparticular, melamine resins. Such crosslinkers do not belong to thebinder solids content of the binder component but together with the freepolyisocyanates of the polyisocyanate crosslinker component they formthe crosslinker solids of the two-component polyurethane clear coatcomposition. The crosslinkers other than free polyisocyanatecrosslinkers may form up to 40 wt.-% of the crosslinker solids of thetwo-component polyurethane clear coat composition. In case the bindercomponent contains no crosslinkers other than free polyisocyanatecrosslinkers, the crosslinker solids of the two-component polyurethaneclear coat composition consist of the at least one polyisocyanate of the1,6-hexane diisocyanate isocyanurate type and the optionally present atleast one polyisocyanate of the isophorone diisocyanate isocyanuratetype.

The two-component polyurethane clear coat kit system of the presentinvention is of particular value, in case its binder component comprisesmelamine resin, in particular, in a proportion corresponding to 20 to 40wt.-% of the crosslinker solids of the two-component polyurethane clearcoat composition. The reason is that two-component polyurethane clearcoat compositions prepared by static mixing of (i) a binder componentcomprising hydroxyl-functional binder and melamine resin and (ii) apolyisocyanate crosslinker component are especially susceptible tointerruptions of or breaks during its spray-application and thedisadvantages associated therewith regarding formation of opticalsurface defects, as explained in the section “Background of theInvention” above.

The binder component contains one or more organic solvents in a totalamount of, for example, 48 to 58 wt.-%. Examples of organic solventsinclude monohydric or polyhydric alcohols, for example, propanol,butanol, hexanol; glycol ethers or esters, for example, diethyleneglycol dialkylethers, dipropylene glycol dialkylethers, in each casewith C1- to C6-alkyl, ethoxypropanol, butylglycol; glycols, for example,propylene glycol and oligomers thereof; glycol ether esters, forexample, ethyl glycol acetate, butyl glycol acetate, 3-methoxy-n-butylacetate, butyl diglycol acetate, methoxypropyl acetate;N-methylpyrrolidone and ketones, such as, methyl ethyl ketone, acetone,cyclohexanone; esters, such as, butyl acetate, isobutyl acetate, amylacetate; aromatic or aliphatic hydrocarbons, for example, toluene,xylene or linear or branched, aliphatic C6- to C12-hydrocarbons.

Additives may be present in the binder component in a total amount of,for example, up to 5 wt.-%. Examples include paint additives such asleveling agents, wetting agents, dyes, light stabilizers, antioxidants,rheology control agents, anti-settling agents, antifoaming agents,adhesion-promoting substances, catalysts.

Examples of transparent fillers that may be contained in the bindercomponent in an amount of, for example, up to 5 wt.-% include silica.

The polyisocyanate crosslinker component has a solids content of 66 to70 wt.-% and a volatile organic content of 30 to 34 wt.-%, wherein thesum of the wt.-% totals 100 wt.-%.

The solids content of the polyisocyanate crosslinker component comprisesa free polyisocyanate solids content consisting of 75 to 100 wt.-% of atleast one polyisocyanate of the 1,6-hexane diisocyanate isocyanuratetype and of 0 to 25 wt.-% of at least one polyisocyanate of theisophorone diisocyanate isocyanurate type, wherein the sum of the wt.-%totals 100 wt.-%. Apart from the free polyisocyanate solids content thesolids content of the polyisocyanate crosslinker component may furthercomprise one or more non-volatile additives as optional constituents. Ifthe polyisocyanate crosslinker component contains no non-volatileadditives, the solids content of the polyisocyanate crosslinkercomponent consists of the free polyisocyanate solids content.

In the description and the claims the term “polyisocyanate of the1,6-hexane diisocyanate isocyanurate type” is used. It meanstrimerization products of 1,6-hexane diisocyanate, for example, trimeric1,6-hexane diisocyanate (1,6-hexane diisocyanate isocyanurate).

In the description and the claims the term “polyisocyanate of theisophorone diisocyanate isocyanurate type” is used. It meanstrimerization products of isophorone diisocyanate, for example, trimericisophorone diisocyanate (isophorone diisocyanate isocyanurate).

The volatile organic content of the polyisocyanate crosslinker componentconsists of one or more organic solvents inert towards free isocyanategroups and, optionally, of up to 2 wt.-% of one or more volatile organicadditives.

Examples of organic solvents which are inert towards free isocyanategroups include, for example, glycol ether esters, such as, ethyl glycolacetate, butyl glycol acetate, 3-methoxy-n-butyl acetate, butyl diglycolacetate, methoxypropyl acetate; ketones, such as, methyl ethyl ketone,cyclohexanone; esters, such as, butyl acetate, isobutyl acetate, amylacetate; aromatic or aliphatic hydrocarbons, for example, toluene,xylene or linear or branched, aliphatic C6- to C12-hydrocarbons.

As already mentioned, the polyisocyanate crosslinker component maycontain one or more additives in a total proportion of, for example, upto 5 wt.-%. Examples include paint additives such as leveling agents,wetting agents, dyes, light stabilizers, antioxidants,adhesion-promoting substances, catalysts.

The binder component and the polyisocyanate crosslinker component of thetwo-component polyurethane clear coat kit system of the presentinvention are stored separately from one another in order to prevent apremature cross-linking reaction.

The binder component and the polyisocyanate crosslinker component of thetwo-component polyurethane clear coat kit system can be statically mixedto form a two-component polyurethane clear coat composition which can beused in a process for the preparation of a clear coat layer of anautomotive OEM multi-layer coating.

The static mixing is carried out in a specified ratio, for example, in amixing ratio corresponding to a 1:0.7 to 1:2 stoichiometry between thehydroxyl groups originating from the binder solids of the bindercomponent and the free isocyanate groups originating from thepolyisocyanate crosslinker component. The two components are mixedtogether making use of a static mixer, for example, a conventionalstatic mixer such as, in particular, a Kenics mixer. Kenics mixers arestatic mixers typically used for automotive OEM industrial productionline coating and they have a length of, for example, 40 to 200 mm. Thestatic mixing allows for a continuous mixing of the two components to bemixed shortly before the two-component polyurethane clear coatcomposition is spray-applied. The two-component polyurethane clear coatcomposition so prepared leaves the static mixer and it is fed to the oneor more spray-application devices.

The two-component polyurethane clear coat composition has a solidscontent in the range of, for example, 45 to 60 wt.-%. The volatilecontent of the two-component polyurethane clear coat composition isformed by (i) the organic solvent(s) originating from the bindercomponent, (ii) the organic solvent(s) originating from thepolyisocyanate crosslinker component and (iii) optionally presentvolatile organic additives.

The two-component polyurethane clear coat composition can bespray-applied on an automotive substrate to be OEM clear coated.

The term “automotive substrate to be OEM clear coated” refers to anautomotive substrate that lacks a clear coat wherein the clear coat isto be applied as an original coating as opposed to, for example, arepair clear coat.

Automotive substrates to be OEM clear coated include in particularprecoated automotive bodies and precoated automotive body metal orplastic parts, the precoatings in each case lacking a final outer clearcoat layer, as explained in the preceding paragraph. In each case theprecoatings themselves comprise an outer pigmented coating layer whichdetermines the color of the finished automotive substrate. Examples ofautomotive bodies include truck and vehicle bodies, for example,passenger car bodies and van bodies. Examples of automotive body metalor plastic parts include doors, bonnets, boot lids, hatchbacks, wings,spoilers, bumpers, collision protection strips, side trim, sills, mirrorhousings, door handles and hubcaps.

The term “finished automotive substrate” refers to an automotivesubstrate provided with an automotive OEM multi-layer coating includinga thermally cured outer clear coat layer applied from a two-componentpolyurethane clear coat composition prepared from the two-componentpolyurethane clear coat kit system of the present invention.

The spray-application of the two-component polyurethane clear coatcomposition formed by static mixing of the binder component and thepolyisocyanate crosslinker component of the two-component polyuretahneclear coat kit system of the present invention may be performed with oneor more interruptions of in each case, for example, 0.5 seconds to 15minutes during the spray clear coating of the automotive substrate to beOEM clear coated with taking the spray-application up again at thatposition on the automotive substrate surface where it had beeninterrupted before. Spray-application is typically carried out byconventional spray-application devices which are generally operated withelectrostatic assistance. Examples of spray-application devices whichare particularly suitable are high-speed rotary atomizers.

The two-component polyurethane clear coat composition may bespray-applied in a dry film thickness in the range of, for example, 20to 60 μm. Then, preferably after a brief flash-off phase of, forexample, 30 seconds to 10 minutes at an air temperature of 20 to 25° C.,the clear coat layer may be thermally cured, preferably, by baking.Thermal curing takes, for example, 20 to 30 minutes and may be carriedout at object temperatures in the range of, for example, 80 to 160° C.

Generally, the mixing of the binder component and the polyisocyanatecrosslinker component of the two-component polyurethane clear coat kitsystem, the spray-application and the thermal curing of thetwo-component polyurethane clear coat are carried out in the context ofan industrial automotive OEM mass production coating process, inparticular, i.e. in an industrial automotive OEM painting facility.

EXAMPLES

Preparation of Clear Coat Layers:

Example 1

1.1) Preparation of a binder component:

A binder component (clear coat base) was prepared from the followingconstituents:

21.5 wt.-% of a 65 wt.-% solution of a saturated polyester resin with ahydroxyl number of 285 mg KOH/g in methoxypropyl acetate,

12.0 wt.-% of a 70 wt.-% solution a saturated polyester resin with ahydroxyl number of 137 mg KOH/g in Solvesso™ 100,

24.0 wt.-% of Setal® 91715 SS-55 from Nuplex Resins (polyester resincontaining sag contol agent),

16.0 wt.-% of Setamine® US 146 BB-72 from Nuplex Resins (partlybutylated melamine resin),

0.2 wt.-% of BYK® 310 from BYK Chemie (wetting additive),

0.9 wt.-% of Tinuvin® 292 from Ciba (HALS light stabilizer),

0.9 wt.-% of Tinuvin® 1130 from Ciba (UV absorber),

13.0 wt.-% Solvesso™ 150,

9.0 wt.-% butyl glycol acetate,

2.5 wt.-% n-butanol.

1.2) Preparation of a polyisocyanate crosslinker component:

68 pbw (parts by weight) of Desmodur® N 3300 from Bayer were dissolvedin 32 pbw of an organic solvent mixture (80 pbw Solvesso™ 100 and 20 pbwbutyl acetate).

1.3) Preparation and application of a two-component polyurethane clearcoat composition:

Using a Kenics mixer (length 150 mm), a two-component polyurethane clearcoat composition was prepared from the binder component from Example1.1) and the polyisocyanate crosslinker component from Example 1.2).Both components were dosed over the Kenics mixer in a mixing ratio of 3pbw of binder component: 1 pbw of polyisocyanate crosslinker component.The two-component polyurethane clear coat composition so preparedexiting the Kenics mixer was directly fed to a high-speed rotaryatomizer and spray-applied in 45 μm dry film thickness onto a 30 cm×60cm automotive body steel test panel provided with electrocoat primer,primer surfacer and black water-borne base coat (predried). Applicationwas performed following a spray path with rectangle profile. After 5minutes flashing off at room temperature the clear coat was bake curedfor 20 minutes at 140° C. (object temperature).

1.4) Example 1.3) was repeated with the difference that thespray-application was interrupted for 20 seconds when the high-speedrotary atomizer reached the middle of the test panel's surface, i.e.after the 20 seconds interruption period the spray application was takenup again at that same position on the panel's surface.

Example 2

Example 1.4) was repeated with the difference that a mixing ratio of 2.5pbw of binder component: 1 pbw of polyisocyanate crosslinker componentwas used.

Example 3

Example 1.3) was repeated with the difference that a differentpolyisocyanate component prepared by dissolving 80 pbw of Desmodur® N3300 in 20 pbw of an organic solvent mixture (80 pbw Solvesso™ 100 and20 pbw butyl acetate) was used.

Example 4

Example 1.4) was repeated with the difference that a differentpolyisocyanate component prepared by dissolving 80 pbw of Desmodur® N3300 in 20 pbw of an organic solvent mixture (80 pbw Solvesso™ 100 and20 pbw butyl acetate) was used.

The multi-layer coatings obtained were tested for their visualimpression. DOI (distinctness of image), long wave (LW), short wave (SW)and dullness (DU) were determined using the measuring device WavescanDOI from BYK-Gardner. In case of examples 1.4), 2) and 4) the testingwas made in a circle (10 cm in diameter) around the middle of the testpanel where the clear coat application had been interrupted for 20seconds. Table 1 shows the results that were obtained.

TABLE 1 Clear coat Visual example impression DOI LW SW DU 1.3) ok 94.30.9  0.5  4.0 1.4) ok 94.1 1.2  6.4  4.2   2) ok 93.8 1.4  5.7  4.9   3)ok 93.1 1.6  6.0  4.5   4) not ok 87.5 3.0 20.3 14.9 ok = homogeneousclear coat layer, glossy surface without defects, not ok = glossysurface with a matting defect in the clear coat layer in the middle ofthe test panel.

1. A two-component polyurethane clear coat kit system consisting of: (1)a binder component having a solids content of 42 to 50 wt.-% comprisingat least one hydroxyl-functional binder and a volatile organic contentof 50 to 58 wt.-%, and (2) a polyisocyanate crosslinker component havinga solids content of 66 to 70 wt.-% and a volatile organic content of 30to 34 wt.-%, wherein the solids content of the polyisocyanatecrosslinker component comprises a free polyisocyanate solids contentconsisting of 75 to 100 wt.-% of at least one polyisocyanate of the1,6-hexane diisocyanate isocyanurate type and of 0 to 25 wt.-% of atleast one polyisocyanate of the isophorone diisocyanate isocyanuratetype, wherein the sum of the respective wt.-% in each case totals 100wt.-%.
 2. The two-component polyurethane clear coat kit system of claim1, wherein the solids content of the binder component has a hydroxylnumber originating from the at least one hydroxyl-functional binder of110 to 160 mg KOH/g.
 3. The two-component polyurethane clear coat kitsystem of claim 1, wherein the at least one hydroxyl-functional binderis selected from the group consisting of hydroxyl-functional(meth)acrylic copolymer resins and hydroxyl-functional polyester resins.4. The two-component polyurethane clear coat kit system of claim 1,wherein the binder component comprises melamine resin.
 5. Thetwo-component polyurethane clear coat kit system of claim 4, wherein thebinder component comprises the melamine resin in a proportioncorresponding to 20 to 40 wt.-% of the crosslinker solids of thetwo-component polyurethane clear coat composition that can be obtainedby statically mixing the binder component and the polyisocyanatecrosslinker component of the two-component polyurethane clear coat kitsystem.
 6. The two-component polyurethane clear coat kit system of claim1, wherein the mixing ratio between the two components corresponds to a1:0.7 to 1:2 stoichiometry between the hydroxyl groups originating fromthe binder solids of the binder component and the free isocyanate groupsoriginating from the polyisocyanate crosslinker component.